Transitional millisecond pulsar PSR J1023+0038
This illustration shows the pulsar PSR J1023+0038 (depicted in white, on the right), which is classed as a millisecond pulsar because of its fast rotation, spinning around its axis within a few thousandths of a second. The pulsar is also part of a binary stellar system, sucking in matter from its companion star (depicted in red, on the left) via an accretion disc (also depicted in red).
In addition to that, PSR J1023+0038 belongs to the rare category of so-called 'transitional millisecond pulsars' that periodically switch between two different modes of emissions – in X-rays and radio waves. Recent observations performed with X-ray observatories in space, including ESA's XMM-Newton, and optical telescopes on ground revealed that this source also exhibits pulses in optical wavelengths and that these occur exactly at the same time as the X-ray pulses.
Astronomers tried to explain these synchronised pulses with existing models, but to no avail, so they developed a new scenario. According to the new model, the pulsar emits a strong electromagnetic wind (depicted as a fuzzy white cloud around the pulsar), which then interacts with the accretion disc around the system.
As the pulsar wind hits matter in the accretion disc, it creates a massive shock, which accelerates electrons in the wind to extremely high speeds. The electrons then interact with the wind's magnetic field, producing powerful beams of synchrotron radiation that can be observed in the optical and X-ray bands at the same time. The location where this interaction is thought to occur is indicated as a bright spot next to the pulsar.
The illustration is not to scale; in reality, the neutron star is much smaller than the companion star.